Detection of hypoxia in microscopic tumors using 131I-labeled iodo-azomycin galactopyranoside (131I-IAZGP) digital autoradiography

Previous studies have shown that tumors less than 1 mm diameter derived from HT29 colorectal cancer cells are extremely hypoxic when grown intraperitoneally or intradermally in nude mice, whereas those of greater size (approximately 1-4 mm diameter) are not significantly hypoxic. The object of this study was to determine if digital autoradiography using the radiolabeled hypoxia imaging tracer iodo-azomycin galactopyranoside (I-131-IAZGP) could detect hypoxia in this model. Microscopic HT29 tumors were grown as disseminated peritoneal disease and intradermally in nude mice. Tumors ranged in size from a few hundred microns to several millimeters in diameter. Animals were intravenously administered I-131-IAZGP and pimonidazole 2 h before sacrifice. Following sacrifice, the intratumoral distribution of I-131-IAZGP was assessed by digital autoradiography and compared with immunofluorescence microscopic images of pimonidazole binding and carbonic anhydrase IX (CAIX) expression. The distributions of I-131-IAZGP, pimonidazole, and CAIX expression were similar. Tumors less than 1 mm diameter displayed high I-131-IAZGP uptake; these tumors also stained strongly for pimonidazole and CAIX. Larger tumors (approximately 1-4 mm diameter) were not significantly hypoxic and had low I-131-IAZGP accumulation. I-131-IAZGP can detect hypoxia in microscopic tumors. Microscopic tumors are useful models for the validation of hypoxia radiotracers, and digital autoradiography is an appropriate technique for studying the distribution of hypoxia radiotracers in microscopic tumors.